1. Field of the Invention
The present invention relates to facsimile communication systems, and more particularly to improvements in the scan interlock circuits thereof.
2. Description of the Prior Art
Transmission of graphic information over large distances is often done by way of telephone networks. Most often such transmission occurs between two transceivers coupled to each end of an established network branch, one transceiver impressing a signal onto the line which is picked up by the other transceiver. The impressed signal typically represents, in electrical equivalent, the local reflectivity of the medium containing the graphical information.
Most often, the medium takes the form of a printed page which is scanned for reflectivity in a line-by-line scan pitch and which at the receiving end provides a control signal for an automatic graphic device. The graphic device, in coherent scan pitch relationship, then duplicates the image.
One such prior system has been described in the U.S. application Ser. No. 391,696, filed Aug. 27, 1973, by Perrault et al, now U.S. Pat. No. 3,914,538, and assigned to the same assignee as the instant application. In the above application, the transmission of facsimile signals is first preceded by a signal group which synchronizes the receiving scan pitch with the transmitting device, equalizes signal intensity at both ends and generally sets up the receiver for regeneration of the graphic image.
In the above context, it is necessary to note that the designation of a receiver or transmitter is purely functional. Generally, the devices at both ends of the line operate as transceivers and are therefore characterized herein according to the mode of operation only.
In the above-referenced application, the scanning of the document or medium is performed by way of a laser beam, the reflection thereof being converted into a video signal. The beam is scanned across the document by an oscillatingly driven mirror and a single photodetector provides the conversion of the beam reflection into an analog signal which varies in intensity with the intensity of the reflected beam. On the receiving end, an autonomous sweep generator provides a deflection signal which, in a similar manner, sweeps a laser beam across a xerographic surface. The sweep or deflection signals in the transmitter and receiver are essentially autonomous. Any small variations in sweep rate or sweep angle quickly show up as a loss of fidelity in the reproduced image.
For this reason, most autonomously scanned systems, including the above-referenced system, require at least a periodic interlock between the transmitter and receiver scanning devices.